“My father made him an offer he couldn’t refuse. Luca Brasi held a gun to his head and my father assured him that either his brains, or his signature, would be on the contract.” — The Godfather (1972)

In the modern global banking system, all banks need a credit line with the central bank in order to be part of the payments system. Choking off that credit line was a form of blackmail the Greek government couldn’t refuse.